Identification of American Herring Gull in a western European context

Identification of American Herring Gull in a western European contextthe size of Europe, that the North Americanpopulation comprises more than just one ‘type’.Indeed, Jonsson & Mactavish (2001) suggestedthat the populations wintering in Newfoundlandand in the Niagara Falls regions, respectively,exhibit sufficiently consistent differences inmorphology and wing-tip pattern to indicate thatthey represent distinct phenotypes. There alsoappear to be differences between East and WestCoast birds (Dwight 1925, Sibley 2000, KlausMalling Olsen pers comm) but most of thepublished information on the ‘West coast type’ israther vague and anecdotal in nature. No doubtfurther sampling and analysis of mtDNA material,combined with more systematic field observationswill improve our understanding of variationwithin smithsonianus.MoultGenerally speaking, moult in smithsonianus issimilar to that of argentatus and argenteus as outlinedby Grant (1986) but with important clarificationsdescribed by Howell et al (1999). Thetendency for some first-years smithsonianus,apparently originating in northern latitudes, topostpone their post-juvenile moult until mid-winter,or even spring of their second calendar-year,mirrors a similar strategy employed by a significantnumber of argentatus (Nikander 1996,Howell 2001, pers obs) but overall, moult is oflittle or no significance when it comes to identifyingindividual herring gulls.Structure and characterSmithsonianus averages a large heavily built bird,appearing similar in size and proportions toargentatus, thus larger than an average argenteus.Many immatures have a demeanour which recallsfirst- and second-year Glaucous Gull due, perhaps,to the combination of a rather thick, long,bicoloured bill and a tendency to have rather uniformlycoloured underparts.Bill structure averages slightly different too,since many smithsonianus have parallel-sidedbills with little gonydeal angle. The bill also averagesslightly longer than in European HerringGulls – but there is overlap.DescriptionsThe following accounts treat each age category,juvenile, first-winter, second-winter and so on upto adult, individually and in that order.Obviously, there is overlap in juvenile and firstwinterplumages, and a certain amount of duplicationin their respective accounts is unavoidable.We restrict ourselves to describing theappearance of smithsonianus in the winterperiod since practically all of our first-hand experience,both in North America and in Europehas been between September and April. Due tothe effects of fading, wear and active moult, thesummer months are considered a much lessrewarding time to study gulls and it may be thatthis is a significant factor in explaining the virtualabsence of summer records of smithsonianus inEurope. However, observations of at least twobirds that over-summered in Cork in 2001(Birding World 14: 224, 2001) suggest that detectionof birds in first-summer plumage may not beas difficult as might have been imagined.The extent of age-related, seasonal, sexual andindividual variation in large gulls is well knownand accounts for many of the associated ageingand identification problems. The potential forindividual variation that exists in most large-gulltaxa and the consequent overlap in the appearanceof many characters has an important bearingon the identification of all ‘out-of-range’ or vagrantlarge gulls, and this is particularly true whenconsidering claims of smithsonianus in Europe.While it would be unreasonable to expect everysmithsonianus recorded in Europe to correspondexactly with the most ‘classic’ examples portrayedin this paper the great majority should, we suggest,be of typical appearance. Although everycase should of course be considered on its merits,problematic birds (ie, birds with a mix of ‘good’and ‘bad’ characters) are arguably more likely tobe unusual-looking European Herring Gulls thanaberrant or atypical smithsonianus. There willalways be cases of contentious birds where it isdifficult or impossible to prove that they are notsmithsonianus, even though birds matching theirappearance would be considered unusual, oreven exceptional, in North America. At the risk oflosing a few records of ‘good’ birds, we areinclined to recommend that, for the time being,only those birds with the strongest credentials beconsidered ‘acceptable’ in Europe.It is impossible in a paper such as this to coverthe vast extent of variation in these gulls. All wecan do is try to define what we consider the mostuseful identification characters for each agegroup and anticipate the most likely sources ofconfusion. We cannot emphasize enough theimportance of acquiring a comprehensive familiaritywith the commoner species of large gulland of always keeping in mind the potential forvariation in these birds.4